Obito has revealed, through taunting madara, that he must use rinnei tensei at cost of his own life to properly revive madara.

After all is said and done, obito having most likely joined the good guys at some point and fighting alongside kakashi with their eyes reaching new powers in a,new way.. madara will remain invincible and undefeated. He just might be too much, this puts obito into a situation only he can handle, he is the only person in the world who can use rinnei tensei. He will cast rinnei tensei without a doubt but not on madara, he will use it to revive hashirama as it is likely the only technique that supercedes the dead demon seal's extreme confinements. The manga has said over and over that hashirama is the only person capable of beating madara. If true then we have more than a slim chance obito will summon him from the death god.

P.s. this theory assumes that rinnegan is exempt and immune to the rules of dead demon sealing (like a master key).